The invention relates to a power actuator comprising a movable actuator member disposed at the rear of a casing wall and having means for generating a magnetic field attached thereto, and a magnetic field sensor adapted to be mounted at the front of the casing wall.
Known from practical use are pneumatic power actuators of the type defined above in the form of so-called pneumatic cylinders. These pneumatic cylinders comprise a piston mounted for reciprocating within a cylinder casing and connected to a piston rod projecting from one end face of the cylinder casing. The cylinder wall of known pneumatic cylinders usually consists of non-ferrous metals such as bronze, aluminum or the like. Provided on the piston of such a pneumatic cylinder is a permanent magnet, the north pole-south pole axis of which is aligned substantially perpendicular to the cylinder wall. The lines of magnetic flux generated by the permanent magnet are able to penetrate the cylinder wall substantially unhindered. Provided on the outer side of the cylinder wall is a hall probe which is subjected to the action of the magnetic field of the permanent magnet when the piston is in a predetermined position. In this manner it is posible to detect the position of the piston in the pneumatic cylinder, so as to permit the operation of the pneumatic cylinder to be controlled or to be stopped at a predetermined position of the piston.
Also known from practical use are so-called hydraulic high-pressure cylinders. Their field of use ranges from elevator platforms to tracklaying vehicles etc. The space for the installation of such hydraulic high-pressure cylinders is often very restricted. It is nevertheless frequently desirable to include means for detecting the position of the piston in a hydraulic high-pressure cylinder. In contrast to pneumatic actuator cylinders, however, the cylinder wall of hydraulic high-pressure cylinders is usually made of steel. Steel is a magnetically conductive material and would therefore act to shield the magnetic field of a permanent magnet attached to the piston. There is also the fact that due to the enormous operating pressures, hydraulic high-pressure cylinders have relatively great wall thicknesses, so that it is no longer possible to detect a magnetic field of sufficient strength for a reliable control function on the outer side of the cylinder wall. It has therefore already been endeavoured to detect the piston position in hydraulic high-pressure cylinders by using other and much more sophisticated methods. All of these methods have in common that they are relatively expensive, and that the instalation space they require on the outer side of the cylinder wall is greater than frequently available.